The present invention relates to well devices employed in the completion of oil and gas wells. More specifically, the present invention relates to a well packer which is retrievably anchored or "set" in a subsurface location within a well casing or other well conduit. When the packer is set, metal locking dogs or "slips" and annular resilient seal elements are extended radially to respectively anchor the packer to the casing and to form a pressure-light seal between the packer body and the surrounding well conduit or casing. A production tubing string connected to the packer conducts well effluents which enter the casing below the packer to the well surface. A variety of well known techniques are employed to set the packer at the desired subsurface location. Manipulation of the tubing string or the application of fluid pressure through the tubing string are examples of two common techniques.
Retrieval of the set packer requires that the radially extended seals and slips be retracted from engagement with the casing. Conventionally, a set packer may be released from its set position by manipulating the tubing string to cause mechanical components in the packer to sever or shift to a position which permits the slips and seals to retract. Once released, the tubing string and attached packer may be withdrawn from the casing.
Manipulation of the tubing string may be difficult or impossible under certain circumstances such as when the well is severely deviated or contains an obstruction. These same conditions, as well as others, may also cause a well packer to set prematurely as it is being run into the well. When premature setting occurs, the gripping force exerted by the set packer slips and seals compounds the difficulty in manipulating or retrieving the packer and attached tubing. The same conditions which make it difficult to retrieve a packer by tubing manipulation also may make it difficult to lock the packer in set position if such locking requires rotation or other surface manipulation of the tubing.
For the foregoing reasons, and others, it is frequently desirable to set, lock and release the packer by non-rotative manipulation of the tubing string. It is customary to design packers which are released by a straight upward pull of the tubing string. Such packers usually include shear pins or other frangible devices which rupture when a sufficient shearing force is imparted through the tubing string. The shear pins must remain intact while the packer is set in its normal operating position but must be capable of rupturing when a predetermined retrieving force is exerted on the tubing string.
The pressure differential acting across the set packer imposes shearing forces on the release shear pins in a straight pull release packer. Accordingly, the shear pins must be of adequate strength to prevent pressure induced shearing which might inadvertently release the set packer. If the pins are too strong, however, an undesirably large upward pulling force may be required to shear the pins for retrieval of the packer. Such large pulling forces, in addition to requiring adequate surface equipment, also impose very strong tension forces on the mandrel itself requiring oversized mandrels with reduced dimension flow passages.
Since the direction of the pressure differential acting across the set packer may be exerted in either direction or may change, it is conventional in some packer designs to employ hydraulic hold-down buttons or dual opposed spreader cones to keep the packer set irrespective of the direction of the pressure differential. In conventional dual cone designs, the seal if frequently above the slips and the pressure induced upward movement of the seal is transmitted past the slips to the lower cone by a tubular mandrel telescoped over the primary mandrel. The need for two mandrels in these packer designs reduced the space available for the packer flow opening. Dual cone designs are nevertheless frequently more desirable than designs using hold-down buttons because of the increased danger of leakage present with such buttons.
Packers having dual flow passges which employ the dual opposed cone designs and a straight pull release usually require a strong primary mandrel which is strong enough to withstand very large tension forces. The need for a strong primary mandrel stems from the fact that all of the differential pressure acting on the set packer and all of the release forces employed to retrieve the packer act through only one of the two mandrels extending through the packer. For these reasons also, the retrieving shear pin required in such packers must also be relatively strong.
Moreover, the strength requirements imposed on the primary mandrel undesirably reduce the size of the flow opening which can be formed through the mandrel. For the stated reasons, hold-down buttons, even with their inherent leakage problem, have been employed more often than opposed cones in dual completion, straight pull release packers.
A variety of prior art packer designs have employed a twin seal, dual cone arrangement to remedy several of the previously noted problems. Examples of such packer designs are shown in U.S. Pat. Nos. 2,765,852; 2,990,882; 3,142,338; and 3,331,440. While these prior art packers have particular desirable features and advantages, they are undesirable in some applications because they either require rotation of the tubing to effect setting or release of the packer or they rely on a pressure differential across the set packer to keep the slips firmly set.